Up to now, a drive control device that controls the driving of a brushless motor having no sensor for detecting a position of a rotor has been known. For example, Patent Literature 1 discloses an example in which the driving of the brushless motor used as a drive source of a fuel pump is controlled by the drive control device.
In the case of a sensorless brushless motor having no sensor for detecting the position of the rotor, the position of the rotor relative to a stator is unknown in a state where the rotor stops before the drive control starts. Therefore, there is a need to start the drive control after the rotor has been forcedly located at a drive start position which is a position of the rotor relative to the stator where the drive control can be started. In the drive control device of Patent Literature 1, the energization of multi-phase windings is switched to detect a position of the rotor relative to the stator. The drive start position is determined on the basis of the detected position of the rotor, and the rotor is located at the determined drive start position. As a result, a time required until the drive control start of the brushless motor is reduced.
In the drive control device of Patent Literature 1, when the position detection and the positioning described above are performed, the rotor is rotated in one direction (forward direction) or the other direction (reverse direction) depending on the position of the rotor relative to the stator at the time of energization. In general, when the brushless motor is used as a drive source of the fuel pump, a shaft of the brushless motor is fitted into a hole of an impeller, and the brushless motor is driven to rotate the impeller. In general, an end of the shaft and the hole of the impeller are D-shaped in a cross section. Moreover, in order to absorb a manufacturing error and an assembly error of the impeller, a predetermined clearance is defined between the end of the shaft and the hole of the impeller. For that reason, when the shaft starts to rotate, a corner of an outer wall of the end of the shaft may collide with a planar part of an inner wall of the hole of the impeller.
When the brushless motor of the fuel pump is driven by the drive control device of Patent Literature 1, the shaft is rotated in the forward direction or the reverse direction every time the energization of the windings is switched, during the positioning prior to the drive control start. For that reason, for example, there is no need to position the rotor, and as compared with a case in which a motor with brush which rotates only in the forward direction is driven, the number of times when the outer wall of the end of the shaft collides with the inner wall of the hole of the impeller, and a collision stress during the collision are increased. This leads to a risk that the impeller may be abraded or damaged. In particular, in idling stop vehicles and hybrid vehicles popular in recent years, because the number of times of turning on and off the fuel pump in a predetermined period of time is large, there is a concern about the promotion of abrasion of the impeller or an increase in the damage due to the collision of the shaft.